In the conventional rotary throttle valve carburetor, moving the rotary throttle valve via a link or lever mechanism, an excessive or undesirably high force may be required to operate the throttle, which can be a limiting requirement in design. Further, a cam for causing the rotary throttle valve to move vertically is also provided between the carburetor body and a lever that actuates the rotary throttle valve. This limits the design freedom as far as shape of the cam and the type of mateiral that can be used.
In a fuel adjustment needle valve of a carburetor proposed in Japanese Patent Application No. 2000-045884, a head portion of an adjustment bolt is received in a vacant portion of a carburetor body, a plug or cap is pressed into the vacant portion and then caulked or the like so that the plug may not be removed, and the engine operator may not adjust the adjustment bolt arbitrarily. The above-described fuel adjustment needle valve of a carburetor poses a problem that the plug or cap is difficult to process and assemble.
As shown in FIG. 16, the conventional throttle valve operating mechanism for a rotary throttle carburetor includes a mounting flange 108 of a lid plate, a holding thread 103 cut in an outer peripheral portion at an end of a cable holder 102 provided with a hexagonal nut 101, and an insert port 104 formed on a central portion at an extreme end of the cable holder 102. For mounting a remote control cable, an end of the inner wire is drawn out of the insert port 104, and the end of the inner wire is retained in a groove 106 of a swivel 105 of a throttle valve lever which engages an idle position bolt 109 when the throttle valve is in its idle position. Then play in the cable is adjusted by rotating the nut 101 and its holding thread 103, and further positioning is done by a lock nut 107. However, it is difficult to mount and retain the inner wire in the groove 106 of the swivel 105 since the surrounding space is narrow and crowded, and the split groove 106 is small. Further, since the insert port 104 is larger in diameter than the inner wire, the outer tube can fall off from the cable holder 102 during assembly.